This lesson will explain how to prepare carboxlic acids using several common methods, including oxidation of primary alcohols and aldehydes, hydrolysis of nitriles, and carboxylation of Grignard reagents.

What Are Carboxylic Acids?

Although carboxylic acids might sound foreign to you, they're actually quite common across many aspects of our lives. Carboxylic acids are used in the manufacture of soaps, pharmaceuticals, cosmetics, candles, rubber and more. Acetic acid, a derivative of carboxylic acids, is one of the main ingredients for manufacturing aspirin.

But carboxylic acids aren't just limited to manufacturing, as they're also important biological molecules. Proteins make up the structure and function of all of our cells and are composed of amino acids, which are monomers. The 'acid' in their name is present due to the carboxylic group in each amino acid. Carboxylic acids are also important metabolic intermediates and waste products in the body.

So, what is a carboxylic acid exactly? Carboxylic acids are organic molecules that have a carbon bound to a hydroxyl group and an oxygen atom. Carboxylic acids are known for their acidic properties and easily donate a hydrogen atom to other molecules.

The structure of a carboxylic acid

Preparing Carboxylic Acids

So how do we make carboxylic acids, in the body and for manufacturing purposes? There are three main ways that carboxylic acids can be synthesized: through the oxidation of primary alcohols and aldehydes, through the hydrolysis of nitriles, and the carboxylation of Grignard reagents. Let's look at each in detail next.

Oxidation of Alcohols and Aldehydes

Primary alcohols are organic molecules where a carbon atom is attached to a hydroxyl group (OH) and another carbon. Primary alcohols are oxidized to carboxylic acids in two steps. First, they are oxidized to an aldehyde, and then again to form a carboxylic acid.

The oxidizing agent used is typically potassium dichromate or sodium dichromate. This ion is able to oxidize the bond between the carbon and the hydroxyl group in the presence of an acid, creating an aldehyde. When excess potassium dichromate is used, the aldehyde can be further oxidized to produce a carboxylic acid.

Although you can also start with an aldehyde, in practice typically primary alcohols are used and are oxidized all the way through to a carboxylic acid. The full reaction is as follows:

3CH2 CH2 OH + 2Cr2 O72- + 16H+ = 3CH3 COOH + 4Cr3+ + 11H2 O

Hydrolysis of Nitriles

Nitriles are a group of organic molecules that have a carbon atom triple bonded to a nitrogen atom. Hydrolysis refers to adding water across a bond, as the prefix 'hydro' means water. So, during this process nitriles are reacted in the presence of acid with water. Oxygen atoms are added in place of the carbon and nitrogen triple bond, and the molecule is oxidized to form a carboxylic acid. The chemical equation for hydrolysis of nitriles with an acid is as follows:

Carboxylation of Grignard Reagents

Carboxylic acids can also be created by reacting Grignard reagents with carbon dioxide, usually in the form of dry ice, in the presence of acid. Grignard reagents are aryl-magnesium halides, or carbon rings attached to a magnesium atom and a halide. The aryl group of the Grignard reagent forms a bond with the carbon in carbon dioxide, and one of the oxygen atoms picks up a hydrogen atom from the acid.

Synthesis of a Grignard reagent and then carboxylation to a carboxylic acid

Grignard reagents have a limited application for synthesizing carboxylic acids, because the reagent itself cannot be made from aryl halides that have certain additional functional groups, such as alcohols, esters, aldehydes and ketones.

Lesson Summary

Carboxylic acids are organic molecules that have a carbon atom bound to a hydroxyl group and an oxygen atom. Carboxylic acids are used in many manufacturing processes, including the production of textiles, pharmaceuticals, cosmetics and more, as well as in biological molecules. Carboxylic acids can be synthesized through three main methods:

Oxidation of primary alcohols, where alcohols are oxidized with potassium dichromate in the presence of an acid.

Hydrolysis of nitriles, where a nitrile is reacted with water in the presence of an acid.

Carboxylation of Grignard reagents, where a Grignard reagent is reacted with carbon dioxide in the presence of acid.

Summary:

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